In a scanning electron microscope, a desired region on a specimen is scanned with an electron beam. The irradiation of the beam produces secondary electrons, which are detected by a detector. The output signal from the detector is supplied as an intensity-modulating signal to a cathode-ray tube which is synchronized with the scan of the electron beam on the specimen. Thus, an image produced from the secondary electrons emanating from the specimen is displayed on the cathode-ray tube.
Usually, such a secondary electron detector comprises a scintillator that produces light in response to incidence of secondary electrons, a photomultiplier, and a light-pipe for guiding light from the scintillator to the photomultiplier. An electron collection electrode to which a high positive voltage is applied is disposed close to the scintillator so that the secondary electrons may efficiently enter the scintillator. This detector is located off the optical axis in the space between the objective lens and the specimen.
Specimens have topographical surface irregularities. Therefore, a specimen surface has portions facing a secondary electron detector. The remaining portions form shade portions as viewed from the detector. Secondary electrons emanating from the facing surface portions are collected to the detector efficiently. As for secondary electrons emitted from the shade portions, however, only some of them can reach the detector. As a result, those portions of the image created on a cathode-ray tube which correspond to surface portions facing the detector appear especially bright, whereas the portions corresponding to shade portions look especially dark. Hence, the resulting contrast is undesirable.
Japanese Patent Unexamined Laid-Open No. 35854/1983 discloses a pair of secondary electron detectors arranged symmetrically With respect to the optical axis of an electron beam. The output signals from the detectors are added to prevent the occurrence of an undesirable contrast as mentioned above.
Japanese Patent Unexamined Laid-Open No. 165357/1984 discloses a technique in which a pair of secondary electron detectors is arranged symmetrically with respect to the optical axis of an electron beam, and in which the ratio of the output signal from one detector to the output signal from the other is taken. Signals are applied to a cathode-ray tube at this ratio to emphasize the surface geometry of a specimen.
In any of the aforementioned prior art techniques, a pair of secondary electron detectors is disposed. Preferably, the detectors are disposed as close as possible to the region of a specimen irradiated with an electron beam, because such arrangement enhances the efficiency of detection. It is necessary that the detectors be located so as not to impede tilt of the specimen.